Traditional field effect transistors (FET's) are familiar conventional devices commonly incorporated as a fundamental building block into the intricate circuitry of integrated circuit (IC) chips. Downward scaling of FET dimensions has improved circuit performance and increased the functional capability of FET's packed on an IC chip. However, continued dimensional reductions may be hampered by the size limitations associated with traditional materials and the costs associated with lithographic patterning.
Carbon nanotubes are nanoscale high-aspect-ratio cylinders consisting of hexagonal rings of carbon atoms that may assume either a semiconducting electronic state or a conducting electronic state. Hybrid FET's have been successfully fabricated using a semiconducting carbon nanotube as a channel region and forming contacts at opposite ends of the semiconducting carbon nanotube extending between a gold source electrode and a gold drain electrode situated on the surface of a substrate. A gate electrode is defined in the substrate underlying the carbon nanotube and generally between the source and drain electrodes. An oxidized surface of the substrate defines a gate dielectric situated between the buried gate electrode and the carbon nanotube. Nanotube hybrid FET's should switch reliably while consuming significantly less power than a comparable silicon-based device structure due to the small dimensions of the carbon nanotube. Such FET's have been successfully formed under laboratory conditions by manipulating single semiconducting carbon nanotubes using an atomic force microscope for precision placement or by coincidental placement of a single semiconducting carbon nanotube between the source and drain electrodes from a randomly dispersed group of semiconducting carbon nanotubes.
The availability of carbon nanotubes and the cost of their synthesis is a primary issue hindering their introduction in various potential mass-produced end products, such as IC chips. A conventional method for synthesizing carbon nanotubes is to deposit a layer of catalyst material on a substrate, which may be patterned to form an array of small dots that operate as seed areas for chemical vapor deposition (CVD) growth using a carbonaceous precursor. The carbon nanotubes grow and lengthen by insertion of activated carbon atoms at each nanotube interface with the catalyst material of the seed areas, which remain affixed to the substrate. As the carbon nanotubes lengthen, the flow of CVD reactant(s) to the seed areas becomes restricted, particularly for dense arrays of seed areas. Specifically, the spaces between adjacent carbon nanotubes open for reactant flow may be narrow. Reactants must flow from the vicinity of the leading free end of the carbon nanotubes through the open spaces to reach the catalyst material to participate in the growth reaction. The flow restrictions slows, and may even halt, nanotube synthesis at the seed areas so that the growth rate slows dramatically, and may cease, with increasing nanotube length.
What is needed, therefore, is a method of synthesizing carbon nanotubes by CVD that is not limited by reactant flow restrictions to the synthesis interface with the catalyst material for seed pads of catalyst material carried by a substrate.